A PFO, illustrated in FIG. 1, is a persistent, one-way, usually flap-like opening in the wall between the right atrium 10 and left atrium 12 of the heart. Because left atrial pressure is normally higher than right atrial pressure, the flap formed by septum primum 14 and septum secundum 16 usually stays closed. Under certain conditions, however, right atrial pressure can exceed left atrial pressure, which creates the possibility that blood could pass from the right atrium to the left atrium through a PFO tunnel 18 and allow blood clots to enter the systemic circulation. It would be desirable to avoid this situation.